This invention regards slew-rate control of output power transistors in general, and the reduction of turn-off delay in particular.
In inductive load drive systems, as those of a DC brushless motor, for example, it is necessary to limit induced disturbances, to minimize both electromagnetic emissions and also noise on power supply lines. Usually, specific circuits for controlling output power transistor slew-rates are used for this purpose.
Slew-rate control is usually implemented by the use of an integrating stage to prevent discontinuities in the load current characteristic so as to drive the output power transistor, as shown in FIG. 1. In this sample circuit, a driving signal DRIVE is fed to an input node A of the integrating stage, which consists of a buffer B1, a feedback capacitor C and a discharge generator I(slew).
The use of such an integrating circuit has several drawbacks. In particular, it should be noted that, when the ratio I(slew)/C (V/.mu.sec) is small, the time taken by output power transistor MP to exit the linear zone and enter the saturation zone of its characteristic, during turn-off, may be too long, and may consequently cause a switching delay that, under certain circumstances, may not be tolerated by the system as a whole.
In order to avoid such a drawback, recourse is commonly made to circuits that permit an "accelerated" slew-rate during an initial phase of a turn-off process, before allowing the discharge to continue with an adequately greater time constant (as determined by the integrating stage), in order to curb disturbances.
An example of such known accelerating circuits is schematically shown in FIG. 2. A logic control signal CONTROL.sub.-- IN is applied to the control terminal of a discharging current generator M1. At the instant t1, when the turn-off of the output power transistor MP is started, the transistor M1 (which may or may not be under current control) is switched-on. This makes the gate of the output power transistor MP to quickly discharge until the input node A of the integrating stage reaches the threshold set by the diode D1 (which may also be replaced by several diodes in series, by a MOS diode, or by another threshold device, as needed). At this point, the contribution to the discharge current provided by the transistor (generator) M1 ceases, and the turn-off process of the MP output power transistor thus continues with a time constant as set by the values I(slew) and C of the slew-rate control, integrating stage. One of the drawbacks of these known solutions is that they rely on a preset threshold to determine the moment at which the discharge generator M1 is switched-off.
Moreover, since it is notorious that the biasing conditions of an output power transistor (MP) strongly depend on load characteristics (e.g. the electrical parameters of the driven inductive load, the working conditions of the driven motor, etc.), it often becomes impossible to set an optimal threshold value when designing the circuit.
Depending on operating conditions, either this difficulty or the approximate character of even the most accurate design choice may often result in failing to optimize switching delay or in an inadequate performance of the slew-rate control circuit. In particular, a retarded turn-off of the M1 switch would make the slew-rate equal to I(M1)/C and, since of course I(M1)&gt;I(slew), the actual slew-rate would be much higher than desired.
An innovative system has now been found which is capable of effectively reducing the turn-off delay of an output power transistor. The system of the invention utilizes a slew-rate control circuit which, unlike known circuits, is capable of adjusting to changing load conditions.
Basically, the turn-off delay reduction circuit employs a "comparator" that senses the potential difference between the output node of the power stage and the input node of a slew-rate control integrating circuit. The comparator produces, on an output node of the comparator, a signal that indicates the state of saturation or of non-saturation of the output power transistor, during a turn-off process. This signal, produced by the comparator, is used to enable and disable the transfer of the driving logic signal IN to the control terminal of the discharge switch of the input node of the integrating stage that drives the output power transistor.
Therefore, the comparator acts as a "sensor", indicating the instant when the output power transistor (preferably a field effect transistor or a bipolar transistor) passes from a "linear" operating zone to a "saturation" zone of its characteristic during a turnoff phase, irrespectively of varying load conditions. Thus, the turn-off delay reduction circuit, object of this invention, exploits a variable and intrinsically optimized threshold for reducing the turn-off delay without affecting the function of the slew-rate control integrating circuit in case of varying load conditions.
According to a disclosed class of innovative embodiments, there is provided: A circuit for driving a power transistor which has a control terminal and two current-carrying terminals, comprising: a first current generator connected to drive a first node, said first node being operatively connected to drive the control terminal of the power transistor; an additional current generator operatively connected, through a respective pass transistor, to drive said first node with a current of the same sign as, and greater magnitude than, the current provided by said first current generator, when said pass transistor is turned on; and a comparator operatively connected to detect the voltage difference between said first node and a current-carrying terminal of the power transistor, and to selectively enable or disable said additional current generator.
According to another disclosed class of innovative embodiments, there is provided: A circuit for driving a power transistor which has a control terminal and two current-carrying terminals, comprising: a first current generator connected to drive a first node, said first node being operatively connected to drive the control terminal of the power transistor; a sensing transistor having a first current-carrying terminal thereof connected to one of the current-carrying terminals of the power transistor, and a control terminal thereof connected to said first node, and a second current-carrying terminal thereof connected to a second current generator; an additional current generator operatively connected, through a respective pass transistor, to drive said first node with a current of the same sign as, and greater magnitude than, the current provided by said first current generator, when said pass transistor is turned on; and logic circuitry connected to drive said pass transistor in dependence on the voltage of said second current-carrying terminal of said sensing transistor.
According to another disclosed class of innovative embodiments, there is provided: A circuit for reducing the turn-off delay of an output power transistor of an output stage comprising: an integrating stage for controlling slew-rate, having an input node driven by a discharge acceleration circuit that consists essentially of a fast discharge switch that can be turned-on by a control signal and turned-off when the potential on the aforesaid input node drops below a certain threshold value, a comparator sensing the potential difference between an output node of the output power stage and said input node of the integrating stage and capable of producing a signal indicative of when the output power transistor reaches or exits saturation during a turn-off phase; and a logic gate having a first input to which said control signal is fed, a second input to which said comparator-produced signal is fed, and an output which is dependent on the logical product of said inputs and is operatively connected to a control terminal of said fast discharge switch; said control signal being transferred to the control terminal of said fast discharge switch when the output power transistor reaches or exits saturation during a turn-off phase.
According to another disclosed class of innovative embodiments, there is provided: A method, implemented in integrated circuit hardware, for turning off a power transistor, comprising the steps of: using a first current generator, driving a first node which is operatively connected to drive a control terminal of the power transistor; in a comparator, detecting the voltage difference between said first node and a current-carrying terminal of the power transistor, and providing an output accordingly; and using said output of said comparator to selectively enable or disable an additional current generator operatively connected to drive said first node with a current of the same sign as, and greater magnitude than, the current provided by said first current generator, when said pass transistor is turned on.